Now - Metal Tubes
June 1935 Radio-Craft

June 1935 Radio-Craft [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

Today as I write this it is New Year's Eve. The year 1935 could be considered 'metal tube's eve' as a new paradigm was about to hit the world of high frequency circuit design. Thentofore[sic], vacuum tubes were almost exclusively encased in a glass envelope. Metal-encased tubes provided, among others, benefits like better heat dissipation, smaller physical size, ruggedness, inherent RF shielding, and lower parasitic values of capacitance and inductance due to smaller plate areas and shorter lead lengths, respectively. The highest barrier to widespread adoption, history would show, was the higher cost of production that made consumer products more expensive at a time when not every household saw the need for a radio or, eventually, a television.

Now - Metal Tubes

As first announced by Radio-Craft, last month, the new all-metal tubes are making their appearance. Many advantages are claimed for the new tubes, as explained.

Since the brief mention made on page 646 of May 1935 Radio-Craft of the new metal tubes, definite information has been released by the General Electric Co. who developed the new method of manufacture for the RCA Manufacturing Co., Inc.

There are many advantages claimed for the new tubes. They are smaller in size, being 1 in. in diameter at the largest point and varying between 5/8-in, to 3 inches high (above the base or sub-panel). They provide their own shielding by virtue of the metal shell and this shell is a better heat conductor and radiator than glass. Because of the method of construction, the leads are shorter, within the tube, which permits greater amplification at the higher frequencies (the inter-electrode capacity is approximately 1/3 of the value for the equivalent glass tube capacity) and the more effective shielding assures greater stability.

Ten types of these metal tubes have been developed, to date, having 6.3 V filaments. Some of these tubes have identical characteristics with present glass tubes (some of them are shown in the photo above, along with their glass twins) though several new types are planned including a double diode and a hexode which is an improved pentagrid converter.

The new tubes have one more base pin than comparable glass tubes, since the metal envelope has become a shield, and provision must therefore be made to ground this envelope. Designers of the tubes have even taken into consideration ease of inserting them in their sockets. The contact pins are all the same diameter and in the center is a longer, larger, insulated keyed pin. By placing this insulated pin in a hole centrally located in the socket and rotating the tube until the key slips into its groove, the tube is quickly and easily inserted. (See "Radio Trends," by Hugo Gernsback. page 69, August 1934 Radio-Craft!)

The mechanical construction of the new tubes is evident from these detailed sketches of a screen-grid type.

The 8-pin base and socket have been standardized for all the metal tubes; when less than 8 pins are needed, they are simply omitted from the tube base and, of course, the corresponding contact on the socket is not wired into the circuit. This is a revolutionary change in tube manufacture.

In appearance the metal tubes are cylindrical in form, some having a reduced diameter at the top. Others, such as a radio-frequency amplifier, have a terminal at the top extremity. Each lead-in wire passes through a tiny bead of special glass that is fused securely within an alloy eyelet, which in turn is welded to the metal container, thus assuring a long life vacuum. This alloy, having substantially the same coefficient of expansion as glass, is known as "Fernico" and is a combination of iron (fer) nickel (ni) and cobalt (co). It was developed expressly for this purpose of a perfect seal in the new tubes.

The inner parts of the tube are first assembled on the steel end plate or "header;" The shell is placed over the assembly and welded to the header at its circumference. This welding is done with Thyratron controlled welders and takes about 1/20-second to complete: about 20,000 to 30,000 amperes flow through the metal when the weld is being made.

During evacuation, the occluded gases in the tubes are removed by heating the entire tube in a gas burner to a red heat. Design engineers claim that the metal envelope helps to "clean up" these molecules of gas, acting as a sort of sponge, thus producing a more complete vacuum.

Elimination of the glass "pinch seal" in which all leads and supports are concentrated in the glass tubes, allows the leads to enter the header of the new tube at the proper points for short direct paths. The metal shields are, of course, much stronger than glass bulbs, and not subject to breakage, while the use of short stiff supports results in less mechanical vibration of the elements.

The familiar metal shield which is necessary with the glass tube in radio frequency portions of a circuit is no longer required with the new tube. The metal envelope itself serves as a shield. And since closer proximity of shield to elements is realized, the shielding is more effective.

No details are available yet, about the price of the new tubes compared to equivalent glass types. However, the methods of making them, and the simplified machines required indicate that the price will be somewhat lower than glass tubes when production reaches its full height. Manufacture of the new tubes will start early in the summer - but it is expected that the first sets to use them will be the fall models of the G.E. line.

In a preliminary folder issued by RCA Manufacturing Co. Inc., six of the new metal tubes are identified with type numbers. The 6A8 is listed as a pentagrid converter (similar to 6A7) - the 6C5 is a triode (similar to the 76) - the 6D5 is a power amplifier triode (equivalent to the 45 but having a 6.3 V., 0.7 A. filament) - 6H6 is a twin diode (this tube has no equivalent in glass tubes) - 6J7 is a triple-grid detector-amplifier (somewhat similar to the 6C6 - though the characteristics are not identical) and the 6K7 is an R.F. pentode with remote cut-off (similar to the 6D6 glass tube).

Posted December 31, 2015